1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to an attachment structure of a PTC (positive temperature coefficient) element in a secondary battery.
2. Description of the Related Art
Unlike the primary battery, the secondary battery may be recharged. Common types of secondary batteries include the nickel-hydrogen battery, the lithium battery, and the lithium-ion battery. The secondary battery may be made into a battery pack and used as the power source for various portable electronic devices such as cellular phones, laptop computers, and camcorders.
Depending on the external shape, secondary batteries may be classified into different types, for example, square, cylindrical, and pouch batteries. With respect to the square secondary battery, this type of battery includes an electrode assembly in which an anode plate and a cathode plate are stacked with a separating plate interposed therebetween. The electrode plate and electrolyte are placed in a metal can that is substantially in the form of a regular hexahedron, and a cap assembly seals an opening of an upper end of the can.
The can may be made of aluminum or an aluminum alloy, which are lighter than and do not corrode as easily as other metals such as iron. The can itself may function as an electrode terminal. For example, an electrode terminal that is insulated from the can in the cap assembly functions as a cathode terminal, and the can functions as an anode terminal.
Furthermore, in secondary batteries as described above, safety devices such as a PTC element, a thermal fuse, and a PCM (protecting circuit module) may be mounted to an exterior of the can before mounting the can in a battery pack. These safety devices are mounted to the anode terminal and the cathode terminal to perform cutoff of current when battery voltage abruptly rises due to an increase in battery temperature or excessive charging/discharging, thereby preventing the battery from exploding.
The above safety devices are connected to the anode terminal and cathode terminal of the battery using a lead plate. The lead plate is made of nickel or a nickel alloy, or stainless steel plated with nickel to thereby have a predetermined hardness and conductivity.
However, many problems result when welding the lead plate made of nickel (or a variation thereof described above) to a bottom surface of the can, which is made of aluminum. Because of the insolubility of nickel and conductivity of aluminum, it is necessary to use laser welding due to difficulties encountered when using ultrasonic welding or resistance welding for connection of these two elements. Laser welding, however, has its own problems. In particular, the heat generated by the laser beam used in laser welding is transmitted to the safety devices compromising the reliability of the devices.
In an effort to overcome these difficulties, U.S. Pat. No. 5,976,729 discloses a secondary battery in which a first lead plate made of nickel is connected, prior to battery assembly, to a bottom surface of an aluminum can. Also, a second lead plate is connected to the first lead plate using resistance welding. These lead plates are used for connection of safety devices.
FIG. 4 is a front view of a conventional secondary battery having a PTC element. A PTC element 1 is fixed to a plate 5 that is attached to a bottom surface 3a of a can 3. Also, a first lead plate 7 extends from the PTC element 1 to a PCM 9, which is mounted to a side surface of the can 3, to interconnect these elements. A second lead plate 11 extends from the PCM 9 to a cap assembly terminal 13 to interconnect these elements. As a result, the PTC element 1 and the PCM 9 are electrically connected to an anode terminal and a cathode terminal of the battery.
With the configuration of the PTC element 1 connected to the bottom surface 3a of the can 3 with the plate 5 interposed therebetween, the PTC element 1 is distanced from the bottom surface 3a of the can 3 by an amount equal to a thickness t of the plate 5. As a result, when the temperature of the secondary battery rises, the transmission of heat to the PTC element 1 occurs through the plate 5. Therefore, some of the heat is dissipated in the plate 5 and not all of the heat of the battery is transmitted to the PTC element 1.
The end result is that the PTC element 1 is unable to perform precise detection of battery temperature. Therefore, current cutoff is performed more slowly than when needed, thus not ensuring battery safety.